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Sight distance available from a point is the

actual distance along the road surface, over

which a driver from a specified height above

the carriage way has visibility of stationaryor moving objects.

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Stopping sight distance (SSD) or the absolute

minimum sight distance.

Overtaking sight distance (OSD) for safe

overtaking operation. Safe sight distance to enter into an

intersection.

Apart from the three situations mentioned

above the following sight distances are alsoconsidered:

Intermediate sight distance.

Head light sight distance.

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Reaction time of the driver

Reaction time of a driver is the time taken

from the instant the object is visible to the

driver to the instant when the brakes areapplied.

IRC suggests a reaction time of 2.5 secs.

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Speed of the vehicle

Higher the speed, more time will be required

to stop the vehicle.

Efficiency of brakes

The efficiency of the brakes depends upon

the age of the vehicle, vehicle

characteristics etc.

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Frictional resistance between the tyre and

the road

The frictional resistance between the tyre

and road plays an important role to bring thevehicle to stop.

No separate provision for brake efficiency is

provided while computing the sight distance.

This is taken into account along with thefactor of longitudinal friction.

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Gradient of the road.

While climbing up a gradient, the vehicle can

stop immediately and vice versa.

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SSD is the minimum sight distance available

on a highway at any spot having sufficient

length to enable the driver to stop a vehicle

traveling at design speed, safely withoutcollision with any other obstruction.

The stopping sight distance is the sum of

Lag distance. Braking distance.

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Braking distance

The distance travelled by the vehicle after

the application of brakes.

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Stopping sight distance = lag distance +

braking distance.

Lag distance

It is the distance travelled by the vehicleduring the reaction time.

If ‘v’ is the design speed in m/sec and ‘t’ is

the total reaction time of the driver in

seconds then lag distance will be v.t metres

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Braking distance

The distance travelled by the vehicle after

the application of brakes.

For a level road this is obtained by equatingthe work done in stopping the vehicle and

the kinetic energy of the vehicle.

If F is the maximum frictional force

developed and the braking distance is l, thenwork done against friction in stopping the

vehicle is Fl = fWl.

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Where

f= frictional coefficient

W= the total weight of vehicle.

l = braking distance.The kinetic energy at design speed is1

2mv2 =

1

2

v2

Hence fWl=

Wv2

2

l =v2

2gf 

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Here l= braking distance, m

v= speed of vehicle , m/sec

f= design coefficient of friction = .4 to .35

depending upon the design speed. g= acceleration due to gravity = 9.8 m/sec2

Therefore stopping sight distance= lag distance

+ braking distance.

ie SD , m = vt +v2

2

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Speed in kmph 20 to30 40 50 60 65 80 100

longitudinal

coefficient of

friction 0.4 0.380.37 0.36 0.36 0.35 0.35

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Ascending gradient

When there is an ascending gradient of say, +

n% the component of gravity adds to the

braking action. The component of gravity acting parallel to

the surface which adds to the braking force

is equal to wsinα = wtanα = wn/100

Equating the kinetic energy to work done:

[fw +Wn

100] l=

1

2

Wv2

g

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l =2

2(+ 

)

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Descending gradient

For descending gradient of – n% the braking

distance increseases

Hence [fw -  Wn100] l= 1

2Wv

2

g

l =2

2(− 

)

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Stopping sight distance for gradient of n%

SD,m= [vt+

  2

2(±  

)]

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